Normal Haematopoiesis
Haematopoiesis is the process by which all blood cells are formed and starts with a pluripotent stem cell. This is a rare cell with a unique self-renewal property which can also differentiate and mature into all cell types.
Haematopoiesis. The haematopoietic stem cell (HPSC) differentiates into common myeloid progenitor (CMP) or common lymphoid progenitor (CLP) cells. CMPs can general all mature myeloid cells while granulocyte-monocyte progenitors (GMPs) or megakaryocyte-erythroid progenitors (MEPs) produce only myeloid/monocytic and megakaryocytic/erythroid lineage cells respectively. B and T lymphocytes and NK cells differentiate from the common lymphoid progenitor (CLP).
Sites of Haematopoiesis
Haematopoiesis starts in the embryonic yolk sac and moves to the foetal liver and spleen after a few weeks (see figure below). The bone marrow then takes over as the main source of new blood cells from 6 months gestation. After birth, active red marrow is gradually replaced by yellow fat marrow so that from age 20, haematopoiesis is concentrated in the axial skeleton and articular ends of the femoral and humerus bones.
Sites of Haematopoiesis (Cellularity in Percentage against Age). In the developing foetus, haematopoiesis occurs initially in the yolk sac, then transitions to the liver. By birth, the bone marrow takes over the majority of the haematopoietic function. Bone marrow cellularity declines with age.
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